Re-draw image using touch method - ios

Question

I use the code below for masking image to remove a part using touch. This works properly using CGContextSetBlendMode.
Now I want to re-draw that image using this touch event. Can you help me for re-draw erased part of the image?
- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
UITouch *touch = [touches anyObject];
currentTouch = [touch locationInView:Second_IMG];
CGFloat brushSize = 35;
CGColorRef strokeColor = [UIColor whiteColor].CGColor;
UIGraphicsBeginImageContext(Second_IMG.frame.size);
CGContextRef context = UIGraphicsGetCurrentContext();
[Second_IMG.image drawInRect:CGRectMake(0, 0, Second_IMG.frame.size.width, Second_IMG.frame.size.height)];
CGContextSetLineCap(context, kCGLineCapRound);
CGContextSetLineWidth(context, brushSize);
CGContextSetStrokeColorWithColor(context, strokeColor);
CGContextSetBlendMode(context, kCGBlendModeClear);
CGContextBeginPath(context);
CGContextMoveToPoint(context, lastTouch.x, lastTouch.y);
CGContextAddLineToPoint(context, currentTouch.x, currentTouch.y);
CGContextStrokePath(context);
Second_IMG.image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
lastTouch = [touch locationInView:Second_IMG];
}

Answer 1

I use the code below for masking image
No, you're not masking at all. You are drawing a new image that lacks the touched area (because you "erased" that part of the image using kCGBlendModeClear). And you are then replacing the image of Second_IMG with this new partially erased image. So there is no "erased part" to "redraw" - you have thrown the information away.
Thus, to do what you are asking to do, you will need first to have access to a copy of the original Second_IMG.image. If you then replace the partially erased Second_IMG.image with the original, all the erased material will magically reappear - because you will return to the image with nothing erased.
But let's say that that isn't your goal: you don't want to bring back all the erased material, but only the material most recently erased. Then you would need to save off each generated intermediate image. You don't want to do this on every touchesMoved:, obviously, because you would end up with thousands of intermediate images. But if you save off the current state of the image on touchesEnded:, for example, then if you want to go back to before the most recent touchesMoved: sequence, you'll be able to, because you saved it on the previous occasion.
It would be simpler, however, if you were really using masking! In other words, if each stroke were expressed as an actual mask layer sitting on top of the image view, then the stroke could be removed simply by removing that mask layer. You would thus be working with masks layers expressing the strokes, rather than replacing the images as you are doing now. You will fine, in any case, that replacing the image repeatedly does not hold up well on the device - it's a very inefficient way to proceed.